Method and apparatus for recovering read errors due to thermal asperities in a disk drive having an MR head

ABSTRACT

The invention provides a method for effectively removing the cause of a thermal asperity (TA) phenomenon occurring at an MR head incorporated in a disk storage system. If a read error occurs in data read by the MR head, a CPU executes a usual read retry operation. If the read error is not removed by the usual read retry operation, the CPU presumes that the read error is caused by the thermal asperity phenomenon occurring at the MR head. Then, the CPU controls the MR head to move to a CSS zone on the disk, thereby executing a TA removal operation so as to remove a fine particle such as dust attached to the MR head. In the TA removal operation, the rotational speed of the disk is reduced so that the MR head can be brought into contact with the disk in the CSS zone.

This is a divisional of application Ser. No. 08/691,113, filed Aug. 1,1996, now U.S. Pat. No. 5,808,825.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a disk storage system particularly used as ahard disk drive and employing a magnetoresistive (MR) head as a readhead.

2. Description of the Related Art

A disk storage system, such as a hard disk drive (HDD), magneticallyrecords data on a disk as a recording medium, and reproduces originalrecord data from data read from the disk, using a magnetic head(hereinafter referred to simply as "head").

In the field of HDDs, a technique for using a magnetoresistive (MR) headas a read head for reading data from a disk has recently been developedto realize high record density. The HDD requires a read/write head forrecording and reproducing data. In general, a head of arecord/reproduction separated type, which consists of an MR head (as aread head) and an inductive head (as a write head) formed integral withthe MR head as one body, is used as the read/write head. In thisrecord/reproduction separated type head, the magnetic gap of the writehead and that of the MR head can be optimized, and hence both recordcharacteristics and reproduction resolution can be enhanced.

The MR head uses an element with a resistance, which is variable inaccordance with a variation in magnetic flux corresponding to avariation in recording magnetic field on a disk. The data reproductionsystem of the HDD converts a variation in MR head resistance to avoltage signal corresponding to a read signal. In the HDD, a head slideron which the MR read head and the write head are mounted moves over thedisk, with a fine flying height, for example, of about 50 nm kept abovethe disk. This being so, only a slight change in flying height may wellcause collision of the MR head attached to the head slider, against aprojection, etc. formed on the disk.

It is confirmed that the temperature of the MR head will abruptlyincrease when the head has collided against the surface of the disk, andthe resistance of the head will accordingly change greatly. Inaccordance with an abrupt change in resistance of the MR head, thewaveform of a read signal output from the MR head varies. Thisphenomenon is called "thermal asperity (TA)" phenomenon. FIG. 10A showsthe characteristics of the TA phenomenon. In this figure, variations inresistance of the MR head appearing when the TA phenomenon occurs areindicated by variations in voltage level. FIG. 10B shows a normal readsignal waveform corresponding to data undisturbed by the thermalasperity FIG. 10C shows a read signal waveform obtained when the thermalasperity phenomenon occurs.

As regards the thermal asperity (TA) phenomenon, see the document"MAGNETIC RECORDING CHANNEL FRONT-ENDS by K. B. Klaassen" (IEEETRANSACTIONS ON MAGNETS Vol. 27, No. Nov. 6, 1991).

When the TA phenomenon has occurred, a read signal with an abnormalamplitude due to the TA, as shown in FIG. 10C, is input to the datareproduction section of the HUD. Since the amplitude of the read signalthus varies significantly, the data reproduction section cannotreproduce data until the amplitude of the read signal returns to anormal level. Moreover, an AGC circuit incorporated in the datareproduction section for keeping the level of the read signal constantwill adversely be affected by the abnormal amplitude of the read signal.As a result, even if the output level of the MR head returns to itsnormal value, the output of the AGC circuit cannot be recovered for acertain period of time corresponding to the time constant of the abruptchange of the signal amplitude.

To solve the above problem, the aforementioned document, etc. proposes amethod for detecting a change in DC level of the read signal output fromthe MR head, and adding a signal waveform of an opposite phase to thelevel-changing read signal to compensate the same. When the TAphenomenon occurs at the MR head, the amplitude of the read signalvaries along an envelope as shown in FIG. 10C, because of abruptincrease in temperature and heat radiation. A DC level detection circuitfor detecting the envelope of the signal waveform can be used as a TAdetection circuit. Such a TA detection circuit is generally employed ina head amplifier circuit in the data reproduction system of the HDD.

As described above, in the HDD using the MR head, it is highly possiblethat the thermal asperity (TA) phenomenon will occur when the MR headcollides against a projection (part of the disk surface itself or a fineparticle attached thereto). The TA phenomenon will cause an abnormalchange in waveform of the read signal output from the MR head, with theresult that it is highly possible that a read error will occur duringdata reproduction. The aforementioned method is proposed to avoid this.The method, however, requires the DC level detection circuit (i.e. TAdetection circuit) and a DC cancel circuit for performing addition of anopposite-phase signal. Furthermore, when in this method, the DC level ofthe read signal has abruptly changed and approached a signal componentband, sufficient DC cancel cannot be performed.

SUMMARY OF THE INVENTION

It is the object of the invention to detect occurrence of thermalasperity (TA) phenomenon at the MR head and remove the cause thereof,thereby effectively removing a read error due to the TA phenomenon andrealizing highly reliable data reproduction.

The invention is applicable to a disk storage system, and comprisesmeans for performing a read retry operation when a read error is foundin data read from the disk by an MR head incorporated therein, anddetermination means for determining that the read error is caused by thethermal asperity phenomenon of the MR head. This determination meanspresumes that the cause is the thermal asperity phenomenon, if the readerror is not removed even after the read retry operation is repeated apredetermined number of times. The invention further comprises recoverymeans for performing an operation for removing the cause of the thermalasperity phenomenon. Specifically, the recovery means moves the MR headto a contact start stop (CSS) zone on the disk, thereby once stopping amotor which rotates the disk, and again starting the rotation of themotor. As a result, a fine particle (such as dust) attached to the MRhead, which may well cause frictional heat and hence the thermalasperity phenomenon, is removed therefrom.

The invention presumes occurrence of the thermal asperity phenomenon ofthe MR head without using a dedicated TA detection circuit. Further, theinvention can remove the cause of the TA phenomenon without using a DCcancel circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a block diagram, showing an essential part of a hard diskdrive relating to the invention;

FIGS. 2 and 3 are flowcharts, useful in explaining the operation of afirst embodiment of the invention;

FIG. 4 is a flowchart, useful in explaining the operation of a secondembodiment of the invention;

FIGS. 5 and 6 are flowcharts, useful in explaining the operation of athird embodiment of the invention;

FIG. 7 is a view, showing a table relating to a fourth embodiment of theinvention;

FIG. 8 is a flowchart, useful in explaining the operation of a fourthembodiment of the invention;

FIG. 9 is a flowchart, useful in explaining the operation of a fifthembodiment of the invention; and

FIGS. 10A, 10B and 10C are views, useful in explaining thermal asperityphenomenon occurring in the conventional MR head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First through fifth embodiments of the invention will be described withreference to the accompanying drawings.

(Structure of HDD)

FIG. 1 shows an essential structure commonly employed in HDDs accordingto first through fifth embodiments of the invention. This structureincludes a head 1 of a record/reproduction separated type, whichconsists of an MR head 2 used as a read head, and an inductive head 3formed integral with the MR head 2 and used as a write head. The head 1has a head slider which flies over a disk 4 at the time of reading orwriting data from or into the disk. The heads 2 and 3 which constitutethe head 1 are mounted on the head slider.

The head 1 is moved by a head positioning mechanism 5 in a radialdirection of the disk as a recording medium, and situated in a targetposition (on a track to be accessed). A single disk or a plurality ofdisks are attached to a spindle motor 6, and rotated at a predeterminedhigh speed. The spindle motor 6 is driven by a motor driver 7. Tofacilitate the explanation below, suppose that the HDD has two heads 1,and a single disk 4 is used. One of the heads 1 is opposed to-onesurface of the disk 4 and the other is opposed to the other surface.

The disk 4 has a data zone 4a for recording data therein, and a contactstart stop (CSS) zone 4b located inside. The CSS zone 4b is a non-datazone, where the head 1 is retreated in contact therewith while the HDDdoes not operate.

The head positioning mechanism 5 is a servo mechanism which mainlycomprises an actuator for supporting and moving the head 1, and a voicecoil motor for driving the actuator. The head positioning mechanism 5 iscontrolled by a CPU 11 as the main control unit of the HDD, so as tomove the head 1 to a target position on the disk 4.

The data reproduction system of the HDD comprises a head amplifiercircuit 8, a read channel 9, and a disk controller (HDC) 10. The headamplifier circuit 8 amplifies a read signal output from the MR head 2and outputs it to the read channel 9. Suppose that the head amplifier 8includes a TA detection circuit 8a which will be described in detail infourth and fifth embodiments. As aforementioned, the TA detectioncircuit 8a is formed of a DC level detection circuit for detecting theenvelope of the conventional signal waveform.

The read channel 9 is a signal processing circuit for reproducing(restoring) data from a read signal output by the MR head 2. Forexample, the read channel 9 is a signal processing circuit which employsa Partial Response Maximum Likelihood (PRML) system. The read channel 9transfers reproduced data (e.g. encoded data of the NRZ system) to adisk controller (HDC) 10.

The HDC 10 has a function for interfacing the HDD with a host computerand a function for controlling data. Specifically, the HDC 10 performsvarious types of control, such as transfer control ofrecord/reproduction data, command processing (including change ofaddress), error check processing of read data, etc.

The CPU 11 is a main control unit in the HDD, and executes a read retryoperation and a thermal asperity (TA) removing operation. Further, theCPU 11 usually performs drive control of a motor driver 7 or the voicecoil motor of the head positioning mechanism 5 via a control circuit (aninterface control circuit consisting of a gate array for outputtingcontrol signals) 12. The control circuit 12 has a function forreceiving, from the read channel 9, servo information necessary for thedrive control of the head positioning mechanism 5, and outputting theinformation to the CPU 11.

A memory 13 is a random access memory (RAM) controlled by the CPU 11,and includes an error counter 13a, a TA flag register 13b and a TAcounter 13c.

(First Embodiment)

An HDD according to a first embodiment of the invention performs a usualread retry operation when a read error occurs, and assumes that the readerror is caused by the thermal asperity phenomenon of the MR head 2 ifthe read error is not removed even after the read retry operation. Then,the HDD performs an operation for removing the TA phenomenon.

The first embodiment will be described with reference to the flowchartsshown in FIGS. 2 and 3.

When the HDC 10 receives a read command from a host computer, the HDDstarts the read operation to read designated data from the disk 4 (stepS1). At the start of the read operation, the CPU 11 sets the errorcounter 13a of the memory 13, and resets the TA flag register 13b (stepS2). The CPU 11 further sets a maximum value Emax as an error countvalue Ec (step S3). The maximum value Emax indicates the maximum numberof occasions wherein the read retry operation or the TA removingoperation is repeated. In other words, the read error count value Eccorresponds to the number of occasions of read retry. If the count valueEc exceeds the maximum value Emax, it is determined that removal of aread error is impossible, and the read error recovery operation isstopped.

In accordance with the read command, the CPU 11 controls the headpositioning mechanism 5 so as to move the MR head 2 to a target positionon the disk 4 (step S4). Specifically, the head (i.e. the slider) 1 ismoved such that the MR head 2 is situated in the target position on thedisk 4 wherein data to be accessed is recorded (i.e. on a target trackincluding a sector to be accessed).

The MR head 2 executes a read operation to read data from the targetposition on the disk 4 (step S5). A read signal output from the MR head2 is amplified by the head amplifier circuit E and input to the readchannel 9, where signal processing is performed to reproduce the data.Suppose that in this embodiment, the TA detection circuit 8a of the headamplifier circuit 8 is not functioning. The read channel 9 reproducesrecord data from the read signal and outputs the reproduced data to theHDC 10.

The HDC 10 performs error check processing to check the reproduced data,and then transfers the checked data to the host computer if it isdetermined that the data is normal (i.e. if the answer to the questionin a step S6 is No). If, on the other hand, it is determined that a readerror occurs (i.e. if the answer to the question in the step S6 is Yes),the HDC 10 notifies it to the CPU 11. The CPU 11 in turn performspredetermined read retry processing.

In a normal state, the CPU 11 changes various read parameters (such as afilter (LPF) parameter, etc.) necessary for the signal processing by theread channel 9 (step S7). Then, the CPU 11 controls the MR head 2 toretry to read data from the target position on the disk (step s8). Atthis time, the CPU 11 increments the count value of the error counter13a. Thus, each time a read error occurs and the read retry processingis performed, the count value of the error counter 13a is incremented.

If the read error is not removed even after the first-time read retryprocessing is performed (i.e. if the answer to the question in a step S9is Yes), the CPU 11 performs the next read retry processing on acontinuous basis. In a step S10, the CPU 11 execute seek offsetprocessing, i.e. processing for correcting the position of the MR head2. In a step S11, the CPU 11 controls the MR head 2 to retry to readdata from the target position. If at this time, no read error occurs(i.e. if the answer to the question in a step S12 is No), the read retryprocessing is finished, and the HDC 10 is shifted to processing fortransferring data to the host computer.

If in this embodiment, the read error is not removed even after the CPU11 performs second-time read retry processing, it determines that theread error is caused by the thermal asperity (TA) phenomenon whichoccurs at the MR head 2 (steps S13, S14).

The inventors of this invention consider that the TA phenomenon may welloccur in a case (1) where a fine particle (dust, etc.) is attached tothe MR head 2, in a case (2) where the MR head 2 contacts a projectionon the disk 4 since the flying height of the head 2 has changed, in acase (3) where the MR head 2 collides with a projection grown on thedisk 4, and in a case (4) where a fine particle (dust, etc.) attached tothe MR head 2 makes its flying height unstable, resulting in collisionthereof with a projection on the disk 4.

(Operation for Removing TA)

The CPU 11 is shifted to processing for removing the TA phenomenon,which is illustrated in the flowchart of FIG. 3. First, the CPU 11checks the error count value Ec of the error counter 13a, and comparesthe error count value Ec (i.e. the number of occasions of read retryprocessing) with the predetermined maximum value Emax (steps S20, S21).If the read error count value Ec exceeds the maximum value Emax (i.e. ifthe answer to the question in the step S21 is No), it is determined thatread error recovery is impossible, thereby stopping the TA removingprocessing and the read retry processing.

If the error count value Ec is lower than or equal to the maximum valueEmax (i.e. if the answer to the question in the step S21 is Yes), theCPU 11 controls the head 1 (the MR head 2) to move to the CSS zone 4b onthe disk 4 (step S22), thereby controlling the driver 7 so as to stopthe spindle motor 6 (step S23). As a result, the rotation of the disk 4stops, and the slider stops floating and contacts the surface of thedisk 4. Accordingly, the MR head 2 contacts the surface of the disk 4.

The CPU 11 then controls the motor driver 7 to start the spindle motor 6(step S24). Accordingly, the disk 4 again rotates, which makes the head1, i.e. the slider, start to float. At this time, the MR head 2, whichcontacts the surface of the disk 4, slides thereon and then floats abovethe disk in accordance with the rotation of the disk 4. If a fineparticle such as dust is attached to the MR head 2, it is highlypossible that the particle is removed in the CSS zone 4b while the headslides therein. Therefore, the aforementioned cause (1) of the TAphenomenon can be eliminated by controlling the stop and start of thespindle motor 6 by the CPU 11.

After executing the TA removing processing, the CPU 11 sets a TA flag inthe TA flag register 13b, thereby executing read retry processing (stepsS25, S26). If no read error occurs during the read retry processing(i.e. if the answer to the question in a step S27 is No), the HDC 10 isshifted to processing for transferring data to the host computer. If, onthe other hand, the read error occurs even after the TA flag, whichindicates the elimination of the TA phenomenon, is set (i.e. if theanswer to the question in a step S28 is Yes), the CPU 11 determines thatread error recovery is impossible, and stops read retry processing.

(Operation for Removing TA according to Second Embodiment)

In a second embodiment of the invention, a TA removing operation asillustrated in the flowchart of FIG. 4 is executed, if the cause of theread error is presumed to be the TA phenomenon as a result of the readretry operation illustrated in the flowchart of FIG. 2.

First, the CPU 11 checks the error count value Ec of the error counter13a, and compares the error count value Ec (i.e. the number of occasionsof the read retry operation) with the predetermined maximum value Emax(steps S30, S31). If the read error count value Ec exceeds the maximumvalue Emax (i.e. if the answer to the question in the step S31 is No),it is determined that read error recovery is impossible, therebystopping the TA removing operation and the read retry operation.

If the error count value Ec is lower than or equal to the maximum valueEmax (i.e. if the answer to the question in the step S31 is Yes), theCPU 11 controls the head 1 (the MR head 2) to move to the CSS zone 4b onthe disk 4 (step S32), thereby controlling the driver 7 so as to reducethe rotational speed of the spindle motor 6 (step S33). As a result, theflying height of the disk 4 reduces, and hence the fine space betweenthe MR head 2 and the surface of the disk 4 further reduces.

The CPU 11 then controls the head 1 to move again to the target positionon the disk 4 (at which the TA occurs), and controls the head 1 to waitthere (steps S34, S35). After a predetermined period of time passes, theCPU 11 controls the head 1 to move to the CSS zone 4b of the disk 4, andcontrols the rotational speed of the spindle motor 6 to return to thenormal value (steps S36, S37). Controlling the head 1 to wait above thetarget position (the TA phenomenon occurrence position) for thepredetermined period time can remove a fine particle (such as dust)attached to the MR head 2 or a projection on the disk 4.

Specifically, since the rotational speed of the spindle motor 6 is lowin the target position on the disk 4, the flying height of the sliderreduces such that the MR head 2 is almost in contact with the disk 4.Therefore, if a fine particle such as dust is attached to the MR head 2,it is highly possible that the particle is brought into contact with thedisk 4 and hence removed from the head. Similarly, if a projection isformed on the disk 4, it may well be brought into contact with the MRhead 2 and hence removed from the disk 4.

Thus, the aforementioned causes (1)-(3) of the TA phenomenon can beeliminated. After executing the TA removing processing, the CPU 11 setsthe TA flag in the TA flag register 13b, thereby executing read retryprocessing (steps S38, S26). If no read error occurs during the readretry processing (i.e. if the answer to the question in a step S27 isNo), the HDC 10 is shifted to processing for transferring data to thehost computer. If, on the other hand, the read error occurs even afterthe TA flag, which indicates the elimination of the TA phenomenon, isset (i.e. if the answer to the question in a step S28 is Yes), the CPU11 determines that read error recovery is impossible, and stops readretry processing.

To reduce the rotational speed of the spindle motor 6 or increase thesame to the normal value, it is not always necessary to move the head 1(the MR head 2) to the CSS zone 4b of the disk 4. However, since theoperation of moving the head 1 to the CSS zone 4b does not requirepositioning control (servo operation), the rotational speed of thespindle motor 6 can be changed smoothly.

(Operation for Removing TA according to Third Embodiment)

In a third embodiment of the invention, a TA removing operation asillustrated in the flowchart of FIG. 5 is executed, if the cause of theread error is presumed to be the TA phenomenon as a result of the readretry operation illustrated in the flowchart of FIG. 2.

First, the CPU 11 checks the error count value Ec of the error counter13a, and compares the error count value Ec (i.e. the number of occasionsof the read retry operation) with the predetermined maximum value Emax(steps S40, S41). If the read error count value Ec exceeds the maximumvalue Emax (i.e. if the answer to the question in the step S41 is No),it is determined that read error recovery is impossible, therebystopping the TA removing operation and the read retry operation.

If the error count value Ec is lower than or equal to the maximum valueEmax (i.e. if the answer to the question in the step S41 is Yes), theCPU 11 controls the head 1 (the MR head 2) to move to the CSS zone 4b ofthe disk 4 (step S42), thereby controlling the driver 7 so as toincrease the rotational speed of the spindle motor 6 (step S43). As aresult, the flying height of the disk 4 increases, and hence the finespace between the MR head 2 and the surface of the disk 4 increases.

The CPU 11 then controls the head 1 to move again to the target positionon the disk 4 (at which the TA occurs), thereby executing usual readretry processing as in the FIG. 2 case (steps S45-S52). At this time, ifthe read error is removed by the usual retry processing, the programproceeds to the processing illustrated by the flowchart of FIG. 6. Inother words, the CPU 11 controls the head 1 (the MR head 2) to move tothe CSS zone 4b of the disk 4, thereby returning the rotational speed ofthe spindle motor 6 to the normal value (steps S53, S54). Thereafter,the CPU 11 sets the TA flag in the TA flag register 13b, followed by thetermination of the processing (step S55).

On the other hand, if the read error is not removed even after the usualread retry processing, the CPU 11 is shifted to processing in the stepS36, et seq. of FIG. 4. Specifically, the CPU 11 controls the head 1(the MR head 2) to move to the CSS zone 4b of the disk 4, and controlsthe rotational speed of the spindle motor 6 to return to the normalvalue (steps S36, S37). Thereafter, the CPU 11 sets the TA flag in theTA flag register 13b (step S38), and performs the processing illustratedin FIG. 3 (steps S26-S28). In other words, if no error occurs after theread retry processing is performed (i.e. if the answer to the questionin the step S27 is No), the HDC 10 is shifted to processing fortransferring data to the host computer. If, on the other hand, the readerror still exists even after the TA flag, which indicates theelimination of the TA phenomenon, is set (i.e. if the answer to thequestion in a step S28 is Yes), the CPU 11 determines that read errorrecovery is impossible, and stops read retry processing.

In the third embodiment, the flying height of the MR head 2 istemporarily increased by increasing the rotational speed of the spindlemotor 6. Thus, if the TA phenomenon is caused by the collision of the MRhead 2 against a projection on the disk 4, the rate of occurrence of theTA phenomenon can be reduced by increasing the flying height. In lightof this, whether or not the read error is removed is determined byexecuting the read retry operation with the flying height of the headtemporarily increased.

If in the third embodiment, the rotational speed of the spindle motor 6is returned to its normal value, the read error due to the TA phenomenonstill exists, since the projection on the disk which is the cause of theTA phenomenon is not removed. However, the CPU 11 can monitor theposition on the disk in which the TA phenomenon occurs, by referring tothe TA flag set in the TA flag register 13b. In this case, the TA flagregister 13b stores the TA flag and a sector address assigned to the TAoccurrence position.

(Operation for Removing TA according to Fourth Embodiment)

In a fourth embodiment of the invention, if the cause of the read erroris presumed to be the TA phenomenon as a result of the read retryoperation illustrated in the flowchart of FIG. 2, the CPU 11 monitorsthe TA occurrence position, and performs predetermined defect processingor processing for warning the occurrence to the host computer. Thisembodiment will be described in detail with reference to FIGS. 7 and 8.

First, when the HDC 10 has received a read command from the hostcomputer, it starts a read operation for reading designated data fromthe disk 4 (step S60). At the start of the read operation, the CPU 11initializes the TA counter 13c of the memory 13 (step S61). The TAcounter 13c is used to monitor the TA phenomenon.

In accordance with the read command, the CPU 11 controls the headpositioning mechanism 5 to move the MR head to a target position on thedisk 4 (step S62). Specifically, the head 1 is moved such that the MRhead 2 is situated in a target position on the disk 4 (a target trackincluding a sector to be accessed). The MR head 2 executes a readoperation for reading data from the target position on the disk 4 (stepS63).

If the CPU 11 detects occurrence of the TA phenomenon at the MR head 2during the read operation (i.e. if the answer to the question in a stepS64 is Yes), it obtains an address corresponding to the position inwhich the TA phenomenon occurs, i.e. a track address and a sectoraddress corresponding to the position on the disk 4 at which the MR head2 has tried to read data (step S65). As is described above, the TAoccurrence can be detected by the presumption method based on the readretry operation. The TA occurrence, however, can be also detected by amethod using a TA detection circuit 8a incorporated in the headamplifier circuit 8 and shown in FIG. 1. The TA detection circuit 8aconsists of a DC level detection circuit for detecting the envelope ofthe read signal wave (see FIG. 10C) output from the MR head 2, anddetects a signal component whose level exceeds a predetermined referencevalue.

When the CPU 11 has detected, on the basis of a detection signal fromthe TA detection circuit 8a, such abnormal level fluctuation in readsignal waveform from the MR head 2 as indicated by the envelope, itpresumes that the TA phenomenon occurs at the MR head 2.

The CPU 11 stores address information (position information) indicativeof the TA occurrence position, and counts the number of occasions, inwhich the TA occurs, by incrementing the count value of the TA counter13c (step S66). At this time, the CPU 11 creates a table as shown inFIG. 7 for monitoring positions in which the TA occurs. The table showsa sector address, a head number, and a TA occurrence count value (Tc),which indicate each TA occurrence position.

If the TA occurrence count value Tc exceeds a predetermined referencevalue (Tmax1) (i.e. if the answer to the question in a step S67 is No),the CPU 11 controls the HDC 10 so as to perform a defect operation (stepS71). The predetermined reference value (Tmax1) enables the CPU 11 topresume that there is a projection on the disk 4 which will cause the TAphenomenon. The HDC 10 sets a flag to indicate, as a defect sector, thesector indicative of the TA occurrence position, and executes the defectoperation for setting a substitutional section (step S71).

On the other hand, if the answer to the question in the step S67 is Yes,the CPU 11 calculates the sum (Tca) of TA occurrence count values in thesame head number (step S68). For example, referring to FIG. 7, the sum(Na+Nb) of a TA count value (Na) corresponding to a sector address (X)and a TA count value (Nb) corresponding to a sector address (Y) iscalculated as regards an MR head 2 with head number 0. Then, the CPU 11compares the calculated sum (Tca) with a predetermined reference value(Tmax2). If the sum (Tca) exceeds the reference value (Tmax2) (i.e. ifthe answer to the question in a step S69 is No), the CPU 11 executes awarning operation to warn the host computer (step S70). From thereference value (Tmax2), the CPU 11 presumes that recording/reproducingof data into/from that data surface of the disk 4 which corresponds tothe MR head 2 with head number 0 is actually impossible.

As described above, in the fourth embodiment of the invention, the TAoccurrence position is monitored, and the defect and/or warningoperation are executed in the HDD, if the cause of the TA occurrencecannot be removed by the methods employed in the first through thirdembodiments, and accordingly the TA occurrence is detected severaltimes. In other words, where the occurrence of the TA phenomenon is nottemporary and may adversely affect the data recording/reproducingoperation of the HDD, such a state is avoided and the reliability of thesystem is maintained.

(Operation for Removing TA according to Fifth Embodiment)

When in this embodiment, occurrence of the TA phenomenon is detected atthe MR head 2 during read operation, error checking and correction (ECC)are performed with respect to data to read. This embodiment will bedescribed with reference to the flowchart of FIG. 9.

First, in response to a read command from the HDC 10, the CPU 11controls the MR head 2 to move to a target position on the disk 4 (stepsS80, S81). The MR head 2 executes the read operation to read datarecorded in the-target position (step s82). A read signal output fromthe MR head 2 is reproduced into read data by the read channel 9, andtransmitted to the HDC 10.

If the HDC 10 detects read error in the read data (i.e. if the answer tothe question in a step S83 is Yes), it notifies the CPU 11 of the error.At this time, the CPU 11 presumes, by the presumption method based onthe read retry operation or using the TA detection circuit 8a, that theread error is caused by the occurrence of the TA phenomenon at the MRhead 2 (the answer to the question in a step S84 is Yes).

The CPU 11 then notifies the HDC 10 of the ECC operation, therebydesignating the TA occurrence position as the error occurrence position(step S85). In the ECC operation, in general, data in a sector in whichthe read error occurs is corrected using an ECC code assigned in unitsof a sector, and corrected data is transmitted to the host computer.Further, in the ECC operation, the error correction ability can beincreased by determining where in the data string the error occurs.Thus, notifying the HDC 10 of the TA occurrence position as the erroroccurrence position enables the same to perform an effective ECCoperation to data in a sector in which the read error occurs.

After the HDC 10 executes the ECC operation, the CPU 11 executes thenormal read retry operation (step S86). After the-read retry operation,the HDC 10 returns the ECC operation wherein the TA occurrence positionis used as the error occurrence position, to the usual ECC operation.

As described above in detail, in the invention, it is presumed duringread operation that the thermal asperity (TA) phenomenon occurs at theMR head 2, and a presumed cause of the phenomenon is removed to therebyeffectively eliminate the TA phenomenon. This means that a read errordue to the TA phenomenon can be reliably removed without using the TAdetection circuit the compensating DC cancel circuit.

In addition, even if the TA phenomenon cannot be removed, it ismonitored and the defect function and the ECC function are effectivelyexecuted. As a result, damage of the system caused by the TA phenomenonis minimized.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative devices, andillustrated examples shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A read error recovery apparatus for use in a diskstorage system having an MR head for reading data from a disk,comprising:first means for detecting thermal asperity phenomenonoccurring at the MR head on the basis of read data read from the disk bythe MR head; second means for determining that position on the disk atwhich the thermal asperity phenomenon of the MR head occurs, if theoccurrence of the phenomenon is detected; third means for counting thenumber of occasions wherein the thermal asperity phenomenon occurs, eachtime the position of the thermal asperity phenomenon is determined;recovery means for removing a read error due to the thermal asperityphenomenon if the counted number exceeds a predetermined referencevalue; means for calculating the sum of the count values of the thirdmeans in each MR head opposed to a corresponding data surface of thedisk, each time the thermal asperity phenomenon occurs; and means forexecuting a warning operation concerning the MR head if the sum exceedsa predetermined reference value.
 2. A read error recovery methodemployed in a disk storage system having an MR head for reading datafrom a disk, comprising the steps of:detecting thermal asperityphenomenon occurring at the MR head on the basis of read data read fromthe disk by the MR head; determining that position on the disk in whichthe thermal asperity phenomenon of the MR head occurs, if the occurrenceof the phenomenon is detected; counting the number of occasions whereinthe thermal asperity phenomenon occurs, each time the position of thethermal asperity phenomenon is determined; removing a read error due tothe thermal asperity phenomenon if the counted number exceeds apredetermined reference value;calculating the sum of count values ineach MR head opposed to a corresponding data surface of the disk, eachtime the thermal asperity phenomenon occurs; and executing a warningoperation concerning the MR head if the sum exceeds a predeterminedreference value.
 3. An read error recovery apparatus for use in a diskdrive having an MR head, comprising:first means for detecting a thermalasperity phenomenon occurring at the MR head on the basis of read dataread from the disk by the MR head; second means for determining thatposition on the disk at which the thermal asperity phenomenon of the MRhead occurs, if the occurrence of the phenomenon is detected; thirdmeans for counting the number of occasions wherein the thermal asperityphenomenon occurs, each time the position of the thermal asperityphenomenon is determined; means for calculating the sum of the countvalues from said third means in each MR head opposed to a correspondingdata surface of the disk; and means for performing a warning operationconcerning the MR head if the sum exceeds a predetermined referencevalue.
 4. An read error recovery apparatus for use in a disk drivehaving an MR head, comprising:first means for detecting thermal asperityphenomenon occurring at the MR head on the basis of read data read fromthe disk by the MR head; second means for determining that position onthe disk at which the thermal asperity phenomenon of the MR head occurs,if the occurrence of the phenomenon is detected; third means forcounting the number of occasions wherein the thermal asperity phenomenonoccurs, each time the position of the thermal asperity phenomenon isdetermined; means for performing a defect operation in that record areaof the disk which corresponds to a position in which the thermalasperity phenomenon occurs, if the count value from said third meansexceeds a predetermined reference value; means for calculating the sumof the count values from said third means in each MR head opposed to acorresponding data surface of the disk; and means for performing awarning operation concerning the MR head if the sum exceeds apredetermined reference value.
 5. A read error recovery method for adisk in a disk drive having an MR head, the method comprising the stepsof:detecting thermal asperity phenomenon occurring at the MR head on thebasis of read data read from the disk by the MR head; determining thatposition on the disk at which the thermal asperity phenomenon of the MRhead occurs, if the occurrence of the phenomenon is detected; countingthe number of occasions wherein the thermal asperity phenomenon occurs,each time the position of the thermal asperity phenomenon is determined;calculating the sum of the count values obtained by said counting stepin each MR head opposed to a corresponding data surface of the disk; andperforming a warning operation concerning the MR head if the sum exceedsa predetermined reference value.
 6. A read error recovery method fordisk in a disk drive having an MR head, the method comprising the stepsof:detecting thermal asperity phenomenon occurring at the MR head on thebasis of read data read from the disk by the MR head; determining thatposition on the disk at which the thermal asperity phenomenon of the MRhead occurs, if the occurrence of the phenomenon is detected; countingthe number of occasions wherein the thermal asperity phenomenon occurs,each time the position of the thermal asperity phenomenon is determined;performing a defect operation in that record area of the disk whichcorresponds to a position in which the thermal asperity phenomenonoccurs, if the count value obtained by said counting step exceeds apredetermined reference value; calculating the sum of the count valuesobtained by said counting step in each MR head opposed to acorresponding data surface of the disk; and performing a warningoperation concerning the MR head if the sum exceeds a predeterminedreference value.